staging: gpib: Declare driver entry points static

Many of this driver's entry points were unecessarily not declared
static.

Remove the declarations from the include file.
Make the declarations static in the .c file.
Remove an uneccessary forward declaration in the .c file.

Signed-off-by: Dave Penkler <dpenkler@gmail.com>
Link: https://lore.kernel.org/r/20250502072150.32714-3-dpenkler@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>